This invention relates to methods for diffusing impurities into a semiconductor.
Among the methods heretofore available for diffusing impurities into a semiconductor substrate through the surface thereof is a method in which the semiconductor substrate is immersed in a liquid containing the impurity and then dried and the dried semiconductor substrate is used as a source for the impurity by subjecting it to a thermal diffusion technique in a closed tube as described in Japanese Patent Publication SHO No. 57(1982)-51968. Other methods include the conventional thermal diffusion method and the ion injection method.
In the production of semiconductors, the procedures for diffusing impurities into the semiconductor constitutes one of the important processes. The technique for impurity diffusion may well be said to have reached a fairly high level. For example, the control of the impurity level and the control of impurity diffusion depth are no longer very difficult tasks.
The methods for impurity diffusion which are now in actual use invariably involve a treatment performed at elevated temperatures in the range of 800.degree. to 1200.degree. C. Consequently, they have not been useful for the production of semiconductor devices whose properties are seriously degraded by exposure to such elevated temperatures. Further, any treatment performed at elevated temperatures has the disadvantages that it requires a good deal of time for raising and lowering the temperature, necessitates use of large scale equipment, and consumes a large amount of energy. More recently, the manufacture of MOS devices and three-dimensional devices has reached a point at which these devices require very thin impurity layers. To meet this requirement, the conventional thermal diffusion methods and the ion injection method are not suitable because they produce excessive diffusion depth.
It is an object of the present invention to overcome the aforementioned disadvantages of the prior art.
Another object of the invention is to provide a method which is capable of forming very thin impurity layers at low temperatures with high repeatability.